The pavement marking industry has long desired transparent microspheres that would be useful as brighter and more durable retroreflective elements in pavement markings. The transparent microspheres now most widely used for pavement markings are made of certain glasses, which are amorphous vitreous materials generally of the soda-lime-silicate type which, although having acceptable durability, have a refractice index of only about 1.5, which greatly limits their retroreflective brightness.
higher-index glass microspheres of improved durability have been taught in U.S. Pat. No. 4,367,919, but even a higher degree of durability has been desired.
A transparent microsphere is taught in U.S. Pat. No. 3,709,706. These microspheres are ceramic microspheres made by sol-gel processes from silica and zirconium compounds. Generally, a sol-gel process is one which converts a colloidal dispersion, sol, aquasol or hydrosol of a metal oxide (or precursor thereof) to a gel, which is a form of colloid which restrains the mobility of the components. The gelling step is often followed by drying and then firing to obtain a ceramic material.
Although the ceramic microspheres taught in U.S. Pat. No. 3,709,706 have good durability, they have diameters smaller than desirable in certain pavement marking applications. Microspheres used in pavement markings generally average between about 150 and 1000 micrometers in diameter, in order to assure that the light-gathering portion of the microsphere protruding from the pavement marking is not obscured by road dirt. The prior art does not teach how to make ceramic microspheres which are both transparent and large enough for these pavement marking applications.
In addition to being rather large and transparent, in order to function in pavement markings, such microspheres need to be resistant to scratching, chipping, cracking, and fracture under the conditions to which they are subjected on the road.